1. Field of the Invention
The present invention relates to a speaker diaphragm which includes at least a layer formed by reinforcing a cloth woven from high strength and high elasticity fiber with resin.
2. Description of the Related Art
Conventionally, to the end of increasing the elasticity of acoustic diaphragm there was a speaker diaphragm formed by reinforcing a cloth of inorganic fiber such as carbon fiber with resin. Those conventional reinforced cloth, however, has relatively high specific gravity and thus it was impossible to fabricate a light acoustic diaphragm with high elasticity. In addition, while the specific elastic modulus is increased, due to the reduced internal loss the material colorations have become trouble at the high frequency region.
Conventionally, there was one material excellent only for a specific acoustic feature, but not material which can satisfy all the acoustic features required for a diaphragm. Accordingly, composite structure diaphragm formed by different characteristics materials have been studied.
In the past few years, a number of new materials have been developed for use in speaker diaphragms. One example is "plasma diamond," which Kenwood announced the development of in 1985. Others include strong of fibers made of materials such as carbon and Kevlar as well as plastics such as polypropylene.
None of these substances satisfy all of the conditions for the ideal diaphragm material which include (1) light weight (2) high sound velocity and rigidity (3) sufficient internal loss. Therefore, efforts are being made to create a balance of the desirable properties of these substances by combining them with other materials.
Composites offer us the opportunity to create diaphragm materials with properties possessed by no one single substance. It is possible to develop materials which balance opposing properties, for diaphragms which are both strong and lightweight, or strong without ringing. We have been conducting research into composite diaphragm materials for many years. Our quest for natural sound reproduction free from unwanted colorations has led to the development of the "HR carbon diaphragm," which features a laminated construction incorporating carbon, which possesses excellent sound velocity and rigidity, and a damping layer to guarantee sufficient internal loss and inhibit the ringing to which carbon is prone. Also notable is the "polygonal carbon ceramic diaphragm" in which the carbon is reinforced by ceramic particles. However, as carbon fiber is the principle material in both of these diaphragms, there are practical limits to how much the weight can be reduced.
Recently, polyethylene fiber is drawing the attention as acoustic diaphragm material due to its high internal loss and good transient characteristics.
For instance, Japanese Laid-Open Gazette No. 58-182994 discloses the diaphragm fabrication method wherein short length polyethylene fibers with the longitudinal wave propagation velocity over 4,000 m/sec are made into a paper-like layer in wet-papering manner. However, since this paper-like layer comprises short length fibers, the tensile elastic modulus in one particular direction of the paper-like layer has disadvantageously become one third the inherent polyethylene tensile elastic modulus.
Japanese Laid-Open Gazette No. 62-157500 proposes the skin layer formation of polyethylene film and composite structure of laminated polyethylene film sheet and fabric. In laminating the polyethylene film on the fabric, due to the weak adhesion of the polyethylene film the lamination structure is very weak in the shear direction. For instance, a large power input to the speaker unit may cause peeling at the interface of the laminated layers due to the amplitude exhaustion.
Most of the conventional acoustic diaphragm for speaker units have been formed from paper pulp. While the paper pulps have an appropriate internal loss, their characteristics are insufficient for elasticity, strength and rigidity so that divided vibrations take place at a low frequency region. Such divided vibration disadvantageously causes peak and dip in the frequency characteristics curve which brings colorations. Conventionally, to the end of improving the paper pulp acoustic diaphragm characteristics, the composite structures of paper pulp layer and inorganic fiber FRP layer such as carbon fiber have been proposed. Even such composite structure, it was difficult to eliminate the peak and dip in the frequency characteristics curve.
Accordingly, the objective of the present invention is to provide an acoustic diaphragm which has appropriately well-balanced characteristics for speaker units.